This invention relates to an antilock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters which include the road surface conditions and the amount of slip between the wheel and the road surface. The braking force increases as slip increases, until a critical value of slip is surpassed. Beyond this critical slip value, the braking force decreases and the wheel rapidly approaches lockup. If the wheel is allowed to lock, unstable braking occurs, and vehicle stopping distance on nondeformable surfaces increases. Thus, stable vehicle braking occurs when wheel slip does not exceed this critical slip value. An antilock control system achieves stable braking and minimizes stopping distance by cycling brake pressure such that braking force is maximized. This is accomplished by first detecting an incipient wheel lock condition, which indicates braking force has peaked and is now decreasing. One criteria that is used to indicate incipient wheel lock is excessive wheel deceleration and/or excessive wheel slip. Once an incipient wheel lock condition has been detected, pressure is relieved at the wheel brake. Upon releasing the brake pressure, the wheel begins reaccelerating toward recovery. The wheel is said to be recovered when wheel slip is reduced to a value below the critical slip value. When the wheel has substantially recovered, brake pressure is reapplied. Reapplication of brake pressure results in the wheel again approaching lockup and the wheel cycle process is repeated. Brake force and vehicle braking efficiency are maximized during braking by cycling the brake pressure around an optimum pressure so that wheel slip is at or near the critical slip value for the particular road surface. Since the brake force is a function of wheel brake pressure and road surface conditions, the critical slip value will change as road surface conditions vary. To optimize vehicle braking during a stop, whether on a changing or uniform road surface, the antilock control system must be able to respond to many road surfaces and cycle brake pressure around the pressure required to produce critical wheel slip for each particular road surface.
If the antilock system slip threshold is lower than the actual critical slip for the road surface, the system will release pressure too soon. This condition is called "under-braked", so named because the system is controlling wheel pressure so as to cycle about a slip value below the actual critical slip value, resulting in below optimal brake pressure and decreased braking efficiency. Conversely, if the system slip threshold is higher than the actual critical slip for the road surface, the system will not release pressure soon enough. This results in an "over-braked" wheel, characterized by longer periods of near-lock wheel instability as the system continuously overshoots the actual critical slip value for the road surface. Over-braking also decreases braking efficiency.
Most antilock brake systems have fixed slip thresholds. Recognizing that a fixed slip threshold may result in either over or under braking of the wheel, some systems attempt to compensate for having fixed thresholds by using deceleration as an indicator of when the actual critical slip for the road surface has been exceeded. A large deceleration value is often said to indicate when the wheel begins operating in the unstable region. Yet, due to the complex dynamic relationship between a braking wheel and a road surface, even using deceleration as a predictive indicator does not completely alleviate the problems of over and under braking associated with using fixed slip thresholds. This is due to the fact that, just as each road surface has a unique value of wheel slip which represents the point of maximum braking efficiency, the tractive characteristics of different road surfaces result in varying deceleration characteristics when the wheel begins locking.
Recognizing that different surfaces have different wheel reacceleration properties, one known system modifies a deceleration indicator based upon an estimate of surface tractive characteristics. This deceleration indicator is used in conjunction with wheel slip to indicate an incipient lock condition, and thereby compensate for the system having a fixed slip threshold which may be either too high or too low in relation to the actual critical slip value for the road surface. This system, described in U.S. Pat. No. 3,717,384, issued Feb. 20, 1973 to John L. Harned and assigned to the assignee of this invention, measures the peak reacceleration of the wheel as the wheel recovers from a lock condition. A low value (&lt;+4 g's) of wheel reacceleration during recovery is classified as indicative of operation on a low coefficient of friction surface. The deceleration indicator is subsequently set to a small value (-1 g) which effectively compensates for the system having a fixed slip threshold which is greater than the actual critical slip value for a low coefficient surface. If, on the other hand, the peak reacceleration during recovery is a high value (&gt;+4 g's), this known method sets the deceleration indicator to a greater value (-2 g's), effectively compensating for a fixed slip threshold which is below the actual critical slip for the road surface.
The use of a deceleration indicator, however, is an indirect and often inaccurate means of adjusting the antilock system's slip threshold so as to detect an incipient lock when the actual critical slip value for the road surface has been exceeded. To consistently modulate wheel brake pressure such that wheel slip cycles about the actual critical wheel slip, the system must be able to directly modify its wheel slip threshold to accurately mimic the unique critical wheel slip for the particular road surface. In doing so, the systems will avoid both the under and over braked conditions inherent in a fixed slip threshold system, while maximizing braking efficiency by cycling wheel slip very closely about the actual critical slip value for the road surface. A key element to determining the actual critical slip for the surface lies in the antilock system's ability to determine the coefficient of friction of the operating surface. Once the characteristics of the operating surface have been determined, the antilock system's next major task is to determine the correct value for the surface-dependent slip threshold. This value should accurately reflect the actual critical slip value for the road surface. By using an incipient lock slip threshold that mimics the actual critical slip for the road surface, both the under and over braked conditions are avoided, and braking efficiency is maximized. Optimal control is thereby achieved by insuring wheel slip is closely cycled around the actual critical slip value for the road surface.